How do I replace multiple lines with single word in file(inplace replace)?

This can be done very easily in perl:

$ perl -i -p0e 's/START.*?END/SINGLEWORD/s' file
$ cat file
My block of line starts from here 
SINGLEWORD
and end to here for example.

Explanation

-0 sets the line separator to null

-p apply the script given by -e to each line and print that line

The regexp modifier:

/s Treat string as single line. That is, change . to match any character whatsoever, even a newline, which normally it would not match.

Why the ?:

By default, a quantified subpattern is "greedy", that is, it will match as many times as possible (given a particular starting location) while still allowing the rest of the pattern to match. If you want it to match the minimum number of times possible, follow the quantifier with a ?.

I was wondering if this is possible without perl, python and others. And I found this solution using sed:

$ sed ':a;N;$!ba;s/START.*END/SINGLEWORD/g' filename

Explanation:

:a create a label 'a'
N append the next line to the pattern space
$! if not the last line, ba branch (go to) label 'a'
s substitute, /START.*END/ by SINGLEWORD,/g global match (as many times as it can)

It was found here.

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